Endosymbiontic Theory: How the Pea Aphid Has Outpaced Science for 100 Million Years

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07/11/2017
By Heike Laue

What if we could absorb superpowers from bacteria?

Image source: Kurkul/Shutterstock.com

Prior to our summer vacation, we would simply ingest a few key bacteria residing in the pelagic black sea devil, so that we may effortlessly dive to a depth of 3,600 meters - much to the amazement of all common recreational divers, who will soon reach the human limit of approximately 40 meters.

Symbionts in action: complete with own genome

Not quite as striking, but nonetheless impressive, are those symbioses that occur naturally in the world of animals and plants. The history of evolution is brimming with examples of symbioses that resulted from a bacterium having been engulfed by a unicellular organism and which has subsequently become active within their prehistoric host as a “symbiont” – the smaller species participating in a symbiotic relationship. Some of these symbionts have managed to develop over time into mitochondria and plastids. As a result, endosymbionts serve as the leading explanatory model for the fact that distinct organizational units exist within eukaryotic cells. But this is not the end of it: they distinguish themselves from their hosts by their own unique genomes, and with the help of ribosomes, they produce their own proteins, which, in turn, feature a composition distinct from that of proteins found in eukaryotic cells. Bacteria with the ability to circumvent the host’s defenses in such a way that it allows them to settle and divert ATP produced by the host cell for their own purposes can be observed in the genus Rickettsia (class: Alphaproteobacteria).

On the life of the pea aphid and its lodgers

In the context of endosymbiosis, some speak of a hypothesis, whereas others consider it a theory. Both points of view share the tangible power of fascination and special scientific potential of this concept. After all, it concerns processes that, in the case of the pea aphid Acyrthosiphon pisum, have been working perfectly well for 100 million years. This tiny light green creature, with an impressive adult height of 3 millimeters, invited into its bacteriocytes bacteria of the species Buchnera aphidicola, which have benefited it with invaluable digestive services ever since. Japanese researchers have conducted a study that was able to show that even the ancestors of the pea aphid were capable of integrating several genes of bacterial origin into their genomes via horizontal gene transfer. Researchers mostly agree on those endosymbionts observed thus far. According to current knowledge, chloroplasts as well as mitochondria are cell organelles which developed through a symbiotic relationship between the host and the symbiont. They are therefore direct descendants of bacteria. Their versatility, too, is illustrated by the example of the pea aphid: through the course of evolutionary processes, the symbionts have by and large adapted to the partnership with their host cells. As such, a closer look at the Buchnera reveals that these have successively ridded their genome of genes that, under their new environmental conditions as “lodgers”, are no longer necessary. But there is more: according to findings by the Japanese scientists, the pea aphid is suspected of having “obtained” and expressed a gene from one bacterium, so that it could be utilized by yet another endosymbiotic bacterium in its own niche.

A recent study has concluded that additional research of such processes within the framework of endosymbiosis would be desirable. For example, laboratory studies could yield deeper insight into the genomes of certain bacterial genera. A human diving excursion into the depths of the North Atlantic would be an even more exciting adventure than it already is today, and such data could make valuable contributions to the analysis of the origin of multicellular life - while at the same time potentially extending our medical arsenal against pathogenic factors.